Lonza is offering $47.20 for each Arch Chemicals share, a premium of 36.7 percent over the US firm's average closing price over the last 30 trading days.
"This transaction represents an excellent strategic and cultural fit. Lonza and Arch Chemicals offer highly complementary products and technologies and together will be the global leader in controlling unwanted microbes," said Stefan Borgas, chief executive of Lonza.
"The business will enjoy a strong platform for accelerated future growth for the benefit of all our stakeholders," he added.
While the merger will require integration costs of $85 million over the next two years, Lonza is aiming to save $50 million a year by the second year of its merger with Arch Chemicals.
It also targets sales growth of $40 million by the third year.
Connecticut-based Arch Chemicals employs 3,000 people and posts annual sales of over $1 billion.